More people who came down with severe acute respiratory syndrome in Block E of the Amoy Gardens apartments also got diarrhoea, creating huge amounts of contaminated faeces that spread the virus through pipes in Hong Kong's biggest outbreak of the flu-like illness, said the health secretary, Dr Yeoh Eng-kiong.
Rats and cockroaches also may have spread SARS, but only incidentally after they picked it up near the apartments, Yeoh told a news conference.
"They were just passive, mechanical carriers," Yeoh said, adding that rats captured by the investigators did not come down with SARS symptoms.
There is no evidence of airborne transmission, Yeoh said.
At least 324 people were infected in Amoy Gardens, where SARS also was apparently spread through person-to-person contact and in common areas, such as elevators, lobbies and staircases, according to the report made by several Hong Kong government agencies.
The SARS outbreak in Amoy Gardens has been the most alarming in Hong Kong, where the disease has infected 1,297 people and killed at least 65. Four new deaths and 29 new cases were reported yesterday.
About 40% of the SARS cases in Amoy Gardens came from one building, called block E, that was evacuated at one point with all of its residents moved into holiday camps that were turned into makeshift quarantine centres.
The disease was brought to Amoy Gardens by an infected man who visited his brother there on March 14 and March 19, Yeoh said. The outbreak peaked in block E on March 24 and three days later spread to residents of other blocks.
Yeoh said that once the virus got into the waste pipe in block E, droplets carrying it apparently moved up through people's bathroom drains when U-shaped water seals dried out. Fans apparently helped suck the virus particles into homes, he said.
One resident of block E said the report's findings sounded right and she called on officials to focus on calming the public and stopping the spread of SARS.
"We have long suspected it could be problems with the sewage system the apartments are so close together," said Anna Yuen, 45, who lives in block E with her husband and three children.
"We were scared at first but after the isolation, we haven't seen any outbreak like what we had before," Yuen said by telephone. "I believe the situation has come under control."
Meanwhile, the world's second most populous nation reported its first SARS case yesterday.